The invention is intended for use in the oil and gas industry, and generally relates to methods and devices that are utilized for stimulating hydrocarbon wells and deposits. More particularly, the invention relates to such methods and device that use metallic plasma-generated, directed nonlinear, wide band and elastic or controlled periodic oscillations at resonance frequencies, and uses the energy released upon plasma formation to quickly alter productivity of said wells and deposits.
The invention further relates to modifying the capacity of such wells, including boreholes and openings, that are production, injection, mature, depleted, waste disposal, conservation, land, on-shore or off-shore. The wells may be oriented at any angle with respect to the earth's surface without horizontal completion. The invention utilizes plasma energy to improve the permeability of said wells and their surrounding matter, optimize the viscosity and/or other physical characteristics of fluids and media, and obtain the enhanced recovery of hydrocarbons and an enhanced intake. In particular, the invention relates to the methods of secondary oil recovery and tertiary oil recovery or enhanced oil recovery (EOR).
The invention also relates to green EOR technologies, because it does not necessitate applying chemical and/or biological agents that are harmful to the environment. In addition, the invention may find useful applications in related types of processes, for example, in increasing the capacity of injection wells, carbon dioxide injection wells, waste disposal wells and wells for the conservation of various materials.
Historically, the average level of oil recovery from a typical well has been approximately 30%. The unrecovered residual oil can be divided into four categories: oil stored in poorly permeable layers and non water-encroached layers—27%; oil in stagnant zones of homogenous horizons—19%; oil in lenses and behind impermeable barriers—24%; and capillary held and film oil—30%.
Oil producers strive to reach the maximal recovery of hydrocarbons from productive deposits at a minimal cost. As numerous oil reservoirs have been depleted worldwide, new advanced methods of enhanced recovery of oil and gas have to be developed in order to extract significant amounts of unrecoverable hydrocarbons left in the reservoirs. Still, no secondary or tertiary recovery-enhancing methods were found to be capable of substantially improving this level of recovery.
Numerous methods and devices for enhancing hydrocarbon recovery have been disclosed in addition to the conventional mechanical ones. The chemical, microbiological, thermal-gas-chemical and similar methods generally rely on using various agent-assisted processes, including: injection of steam, foam surfactants and/or air, the latter being accompanied by low-temperature or high-temperature oxidation, in situ formation of emulsions, directed asphaltene precipitation, chemical thermal desorption, selective chemical reactions in light oil reservoirs and heavy oil deposits, chemical agent-assisted alterations of phase properties, including wettability and interfacial tension, and alkaline-surfactant-polymer flooding, to name a few.
Alternatively, EOR can be achieved through stimulating the well/deposit permeability and improving oil mobility by means of agent-free apparatuses generally related to the following types of the equipment: ultrasonic, acoustic, electrohydraulic, electric hydro-pulse and electromagnetic emitter devices, as well as devices that are combinations thereof.
It has been reported that the oscillations supplied by an ultrasound (frequency >20 KHz) source can improve the permeability of much of the porous media surrounding the well. Accordingly, high-power ultrasonic apparatuses are used for the removal of barriers that block oil flow into the well, the reduction of particle clogging near the well bores and cleaning/clearing the near wellbore regions in the producing formations that exhibit declining production as a result of mud penetration, depositions and other undesirable processes. However, EOR through ultrasound does have a major disadvantage in that high-frequency waves are rapidly attenuated in naturally existing porous media, which results in a rather limited influence on the formation and bottom-hole zone. This leads to limited intensification of inflows and a moderate increase in oil recovery.
Most devices for EOR through ultrasound are designed for insertion into the wells/boreholes. All of these devices comprise an ultrasonic transducer and ultrasonic emitter(s) powered through a logging/power cable. The ultrasound treatment of the wells/boreholes focuses on an improvement in the filtering properties of productive intervals and is performed point by point, with the neighboring points usually being distanced between 0.5-1 meter from one to another. The efficiency of EOR through ultrasound is assessed based on the inflow profile-stimulation profile data. The ultrasound treatment is effective in approximately half of the cases. The improved permeability imposed by the ultrasound EOR is not permanent, although it may last for months.
It has been observed that both an enhancement of oil recovery and an increase in well intake were achieved through the action of seismic waves originating from earthquakes and waves that resulted from various human activities. Moreover, oil production can be promoted by sending seismic waves across a reservoir to liberate immobile oil patches. Seismic waves are mechanical perturbations that travel through the Earth at a speed governed by the acoustic impedance of the medium in which they are propagating. Apart from the ultrasonic waves, which are capable of affecting the local regions, the seismic waves may stimulate a whole reservoir, inducing a large-scale effect due to their low attenuation.
Low-frequency elastic waves of a low intensity can significantly increase the flow rate of yield-stress fluid under insignificant external pressure gradients. They promote entrapped non-aqueous liquid bubble mobilization and non-aqueous phase liquid transport in porous media by lowering the threshold gradient required for the fluid's displacement.
The propagation of surface acoustic (frequency is 20 Hz-20 KHz) waves depends on elastic and piezoelectric nonlinearity, and is characterized by a frequency shift due to external static stresses and electric fields. Nonlinear wave propagation is affected by the difference between non-dispersive and dispersive systems, with the two types being able to occur in electroelasticity. In dispersive media, self-focusing, self-modulation, envelope solitons, and the attenuation of surface waves takes place due to coupling the thermal and quantum fluctuations.
Heterogeneous porous reservoir media are nonlinear due to the plurality of both micro- and macro-defects, as well as grain-to-grain contact surfaces comprising multiphase fluids. In the porous reservoir materials, quasi-static and dynamic responses are mostly determined by the reservoir fluids. The nonlinear effects can significantly affect the efficiency of oil recovery, because oil trapping depends on permeability. In the low-frequency range, capillary forces and nonlinear rheology are the main mechanisms of seismic/acoustic stimulation. Nonlinear sound scattering by spherical cavities in liquids and solids and the stress-deformation in solids/media with micro plasticity, which are affected by wide-band random excitation and exhibit properties of hysteresis, are analyzed using multi-degree-of-freedom models. The interaction of acoustic waves in micro inhomogeneous media is stronger when compared to that in the conventional homogeneous media, which was observed with ground species, marine sediments, porous materials and metals.
Oil trapped on capillary barriers can be liberated when seismic amplitudes that exceed a certain threshold are followed by oil transfer under background pressure gradient(s). The movement is further enhanced by droplet coalescence. The effective force added by seismic waves to the background fluid-pressure gradient is estimated using poroelasticity theory. The fluid's pore-pressure wave and the matrix elastic waves are responsible for the increase in oil mobility. The rock-stress wave is the more efficient energy-delivering agent compared to the fluid pore-pressure wave in a homogeneous reservoir.
EOR through seismic vibration-assisted mobilization of oil has not yet been fully studied. In practice, seismic waves are generated using arrays of powerful sources placed on the earth's surface. The level of the introduced vibro-energy affects both residual oil saturation and relative permeability in the porous medium. Oil mobilization in homogeneous and fractured reservoirs can be altered via a fluid's oscillation in a well. EOR in the fractured reservoir's matrix zone and cross-flow induced by vibrations improves the imbibition of water into and expulsion of oil out of the matrix zone.
The electrohydraulic method allows the enhancement of oil recovery by means of the restoration of filtration properties of a productive layer. The method comprises the generation of shock waves in a fluid as the result of the application of very brief, but powerful electrical pulses followed by the occurrence of shock waves with acoustical and hypersonic velocities.
U.S. Pat. No. 6,227,293 to Huffman et al. and U.S. Pat. No. 6,427,774 to Thomas et al. disclose processes and apparatuses for coupled electromagnetic and acoustic stimulation of oil reservoirs using pulsed power electrohydraulic and electromagnetic discharges. The combination of electrohydraulic and electromagnetic generators causes both the acoustic vibration and electromagnetically-induced high-frequency vibrations over an area of the reservoir. The effective range of the stimulation is limited to 6000 feet. In addition, the design of these combined generators is complex and they have sizeable dimensions, which limits their use with conventional boreholes: in some cases an additional well needs to be drilled for the placement of the generator.
Another approach illustrated in U.S. Pat. No. 6,499,536 to Ellingsen teaches a method that includes injecting a magnetic or magnetostrictive material through an oil well into the oil reservoir, vibrating the material with the aid of an alternating electric field and removing oil from the well. The method requires the use of additional materials and has disadvantages associated with the introduction of these solid materials into the productive layer, including a possible decrease in permeability.
A borehole acoustic source for the generation of elastic waves through an earth formation and the method of using it is disclosed in U.S. Pat. No. 7,562,740 to Ounadjela, and can be utilized for measuring the geological characteristics of the underground media surrounding the borehole. The method relies on using frequencies up to at least 1 KHz and is a geophysical research method and is not intended for EOR.
U.S. Pat. No. 6,597,632 to Khan discloses a method for determining the location and the orientation of open natural fractures in an earth formation by analyzing the interaction of two high-frequency and low-frequency seismic signals recorded in another wellbore. In this method, the low-frequency signal is transmitted from the earth's surface and the high-frequency signal is transmitted from the wellbore. The compression and rarefaction cycles of the lower frequency signal are used to modulate the width of the open fractures, which changes their transmission characteristics. As a result, the amplitude of the high-frequency signal gets modulated as it propagates through the open fractures. This method is applicable for subsurface fracture mapping using nonlinear modulation of a high-frequency signal, and is not intended for use with EOR purposes.
A method and apparatus for blasting hard rocks for the fracturing and break-up of the rock using a material ignited with a moderately high energy electrical discharge is disclosed in U.S. Pat. No. 5,573,307 to Wilkinson et al. The two electrodes of the reusable blasting probe are in electrical contact with a combustible material such as a metal powder and oxidizer mixture. Electrical energy stored on the capacitor bank ignites the metal powder and oxidizer mixture causing an increased dissipation of heat generating high-pressure gases fracturing the surrounding rock. Wilkinson teaches the utilization of oxidizing chemicals for rock fracturing, but not for the stimulation of oil production.
Yet another apparatus for generating pulsed plasma in a fluid is described in U.S. Pat. No. 5,397,961 to Ayers et al. A high-energy pulse is supplied to spaced electrodes for creating a spark channel and initiating the plasma. The pulse-forming network generates a pulse with the duration of 5-20 microsecond and gigawatts of power.
U.S. Pat. No. 5,425,570 to Wilkinson discloses a method and apparatus for blasting rocks with plasma. A capacitor bank is used for storing an electrical charge, which is coupled with an inductance that delivers the electric charge as a current through a switch to an explosive helically wounded ribbon conductor. The ribbon's dimensions correspond to the ratio of the inductance to the capacitance in order to ensure the efficient dissipation of an optimal amount of stored electrical energy.
It shall be noted that a number of EOR methods currently utilized in practice are based on linear dependencies/phenomena. However, the linear dependencies in nature can be viewed as the exceptions, rather than the rule due to the numerous possible combinations of various dependencies resulting in very diverse and uniquely complex effects.
For example, in the 1950s, a deviation from a phenomenologically derived constitutive Darcy's law, which is used to describe oil, water and gas flows through petroleum reservoirs, was observed and the nonlinear filtration law was discovered. The filtration rates of oil and oil-containing fluids vary greatly, depending on viscosity, pressure gradient and other conditions.
The multiphase systems and their nonlinear wave dynamics are of growing importance for state-of-the-art industrial applications, including: acoustics and shock waves in homogenous gas-liquid and vapor-liquid mixtures, dynamics of gas and vapor bubbles, wave processes in gas-liquid systems and on the interface of two media, wave propagation in a liquid medium with vapor bubbles, wave flow of liquid films and calculation of wave dynamics in gas-liquid and vapor-liquid media. Since a productive deposit is a dissipative medium with a combination of nonlinear oscillations in a wide range of frequencies, it is impossible to explain the origin of the processes by an occurrence of forced periodic wide-band oscillations using the general laws of physics. Nonlinear phenomena violate the principle of superposition. The response of a nonlinear system to a pulse with a certain length is not equal to the sum of its responses to shorter pulses with a duration of tens of microseconds. For instance, the system's response to two consecutive pulses with the duration Δt each differs from its response to a single pulse with the duration 2Δt.
The interaction of the wide-band, periodic, directed and elastic oscillations generated by the ideal nonlinear plasma source with a nonlinear, dissipative and non-equilibrium medium results in nonlinear wave self-action at the basic frequency. In this case, wave amplitude and frequency change depending on the intensity of the wave in the form of a single quasi-harmonic; the amplitude and the phase of this quasi-harmonic slowly change over time and space, as a result of the nonlinearity. Thus, the self-modulation effect is observed in the disturbed nonlinear system. Due to periodic pulse impact, the phase transition starts manifesting the transformation from one state to another. This transformation is accompanied by an increase in phase transition temperature, starting with bubble nuclei formation, and heat exchange. The periodic impact leads to the development of resonance oscillations at quasi-harmonic frequency under these conditions. The harmonic low-frequency oscillations last for a long period of time following impact termination.
Presently, with the cost of oil rapidly rising, it is exceedingly desirable to reduce time and to lower energy consumption in order to secure a profit margin that is as large as possible. However, prior art techniques do not offer the most efficient method of EOR in the shortest amount of time possible, especially in depleted and mature wells. Accordingly, there is a pressing need for a process and a device that adequately addresses the above described necessities in an advanced EOR, and will allow the enhancement of oil and gas recovery with minimal time for treatment and energy cost that would result in the improved characteristics of the wells/boreholes and their surrounding media. Such a process and device shall be capable of increasing both the recovery of hydrocarbons from deposits and the intake capacity of injection wells and that of waste storage wells. The advanced, compact and highly efficient device is particularly needed in the light oil production fields, where the depletion is a key concern. Several other objectives and advantages of the present invention are:                (1) To provide a device for treating wells/boreholes in an expedited manner with optimized energy costs;        (2) To ease operation, improve efficiency and reduce space taken up by the equipment;        (3) To provide a device for use with aggressive well media for any required period of time;        (4) To provide conditions for altering the permeability of the media surrounding the well and the mobility of associated fluids by passing through the surrounding media filled with the fluids the metallic plasma-generated, directed, nonlinear, wide-band and elastic oscillations at resonance frequencies following the controlled explosion of a calibrated conductor in the in-well plasma source;        (5) To provide conditions for the gradual, multi-step alteration of the medium's permeability and fluid mobility by subjecting the well's surrounding media and constituents of said fluids to the first shock wave event followed by subjecting the disturbed well surrounding media and affected constituents of said fluids to the second shock wave, etc.        (6) To provide a device for manipulating the capacity of land, onshore and offshore wells of predominantly vertical orientation with respect to the earth's surface or sea bottom and their surrounding media;        (7) To provide conditions to obtain capacity improvements resembling those of hydro cracking;        (8) To produce oscillations throughout the media/reservoir/deposit for a period of time sufficient for the efficient recovery of unrecovered hydrocarbons;        (9) To provide the device, wherein two or more plasma sources can be employed.        
The present invention fulfills these needs and provides other related advantages.